/ ' - V y / OOQ O

PINSTECH/NMD-112

EXTRACTION OF ZIRCONIUM FROM ZIRCON. (A NEW PROCESS)

K.A SHAHID DANIEL SAhED SAIDA JAN ARIF MASOOD JAVED AKHTAR

NUCLEAR MATERIALS DIVISION Pakistan Institute of Nuclear Science and Technology

Nilore, Rawalpindi.

October, 1985

PINSTECH/NMD-112.

Extraction of Zirconium from Zircon. (A New Process).

by

K.A. SHAHID DANIEL SAEED 5AIDA JAN ARIF MASOOD JAVED AKHTAR

Nuclear Materials Division Pakistan Institute of Nuclear Science & Technology

P.O. Nilore, Rawalpindi. October, 1985.

CONTENTS

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ABSTRACT INTRODUCTION 1 EXPERIMENTAL 3 2.1 Materials and Methods 3 2.2 Experimental Results 4

2.2.1 Caustic Fritting with Magnesiumhydroxide-carbonate 4

2.2.2 Caustic Fritting with Magnesium Oxide 4

DISCUSSION 5 SUMMARY AND CONCLUSIONS 6 ACKNOWLEDGEMENT 8 REFERENCES 9 TABLES 10-12

ABSTRACT

Caustic fritting was done in the presence of magnesium hydroxide carbonate and magnesium oxide. These additives decrease the amount of nitric acid soluble silica less than 1000 ppm. Caustic fritting without any additions results in some nitric acid soluble silica, which is removed by dehydration with concentrated sulphuric acid. The present process eliminates the sulphuric acid dissolution and subsequent precipitation with ammonia which involves difficulty in washing of sulphate ions from the geletinous hydroxide cake and hence makes the direct nitric acid dissolution an attractive option. The effect of excess magnesium was studied and extraction does not seem to be effected by excess magnesium. How the reaction proceeds is not exactly understood but a probable mechanism has been postulated.

INTRODUCTION

Due to the low thermal absorption cross-section and good mechanical and corrosion resistant properties at higher temperature, zirconium alloys are preferred for cladding and structural materials in thermal neutron power reactors.

Hafnium occurs invariably with zirconium in nature. While the chemical properties are very similar yet the thermal absorption cross-section of Hf is quite high.

The zirconium to be used in nuclear reactors should therefore be hafnium free. A typical chemical specification for nuclear grade zirconium sponge is given in Table I .

2 A variety of processes have been developed for production of hafnium free zirconium. In each of these processes, various head-end steps for hafnium-zirconium separation processes and metal production techniques have been employed; Nandi has reviewed different laboratory techniques for solvent extraction.

Out of the different processes three that have been used on industrial scale are:

4 1. Fractional crystallization of double flourides .

5 2. Solvent extraction of the thiocyanates by hexone .

3. Solvent extraction of the nitrates by tributyl phosphate (TBP).

The latter method has been adopted for the production of hafnium free zirconium in NMD laboratories. The method has been described by Cox et al. For TBP solvent extraction process, the zirconium nitrate solution is prepared by caustic fritting of zircon sand. Zircon is fused with caustic in 1: 1.1 ratio at 500-650°C to produce a frit of sodium silicate and sodium zirconate.

4 NaOH + ZrSiQ4«, »Na 2 Zr03 + Na 2Si0 3 + 2 H O.

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The frit is then washed with cold water to remove soluble sillicates and excess caustic.

To provide a suitable feed for solvent extraction traces of silica from washed frit are removed by dissolving it in sulphuric acid. The solution is filtered to remove silica and unreacted zircon. The zirconium sulphate solution obtained as filtrate is precipitated as zirconium hydroxide with ammonium hydroxide. Zirconium hydroxide so obtained is dissolved in nitric acid to form zirconium nitrate. To obtain a satisfactory separation factor for zirconium and hafnium in TBP solvent extraction, it is desirable to reduce the silica contents in nitrate solution prepared from

t caustic fritting, to less than 1000 ppm'. Although much of o

silica is reported to be removed during the direct dissolution of the frit in hot concentrated nitric acid. Silica floe so produced was found to be in such a colloidal state that the filtration of the solution was rather difficult, elimination of silica being incomplete.The dissolution of the washed frit directly in nitric acid is not always successful. The feed solution prepared for subsequent solvent extraction in nitric acid has been reported erratic in behaviour and a consistently suitable feed is not obtained.

The dissolution of the caustic frit in sulphuric acid and subsequent conversion to nitrate solution after precipitation with ammonia, reduces the silica contents in the feed solution to an acceptable level. The sulphate ions so introduced however have a very deletrious effect on the distribution coefficient of zirconium and separation factor of zirconium and hafnium in TBP solvent. It is possible to lower sulphate ions content to acceptable 9 levels (sulphate/zirconium = 0.17) by prolonged washing of the zirconium bhydroxide precipitates before dissolution in nitric acid. However, this is a time consuming process and adversely effects the process efficiency. The use of concentrated sulphuric acid for silica removal also results in increased corrosion problems an well as more hazardous working conditions. Efforts have therefore been made to remove silica by other methods, most note worthy of them has been the u^e of gelatin for silica removal. The method though reported to be effective, makes the process a litt]e combersome and has not been adopted commonly.

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The present study is aimed at charging the fritting process in a way, that the direct dissolution of the frit in nitric acid yields a nitrate solution with acceptable silica level, without the use of sulphuric acid or gelatin. It was considered that an alkaline earth oxide if added to the caustic fritting composition, might bind silica in such a way that makes it insoluble in nitric acid. In the present studies, fritting of zircon sand was carried out with a mixture of sodium hydroxide and maganesium oxide and magnesiumhydroxidecarboaate (which yield magnesium oxide on heating).

2. EXPERIMENTAL

2.1 Materials and Methods

Pak zircon was used as a starting material for all the experiments. The composition and size deistribution of the ore are given in the Table II.

The fritting was done without screening or any other pretreatment of the sand. The laboratory reagent magnesium oxide (Heavy) from BDH Ltd, England magnesium-hydroxide-carbonate (Heavy) from MERCK Ltd. and sodium hydroxide (98%) in pallets from Fluka-Garantie were used. Commercial nitric acid (12.6 N) was used throughout the investigations. Magnesiumhydroxidecarbonate/magnesium oxide was mixed with zircon sand and fused with sodium hydroxide in a S.S. crucible made from a 4.5 cm dia pipe welded shut at one end in a pit furnace. The melt boiled for nearly five minutes. Post heating time of 120 minutes was given to the samples.The resulting frit was a granular powder which 'flowed' from the cruicible very easily. It was then washed with cold water to remove excess caustic and soluble silica.

The dried washed frit was dissolved in hot nitric acid and filtered to remove acid insoluble silicates and unreacted zircon. The cake was then washed with 10% HNO,. The filtration was fast.

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Zirconium concentration in the solutions were determined by EOTA titration and SiO, concentration was deter-

11 mined by spectrophotometry methods

2.2 Experimental Results

Caustic fritting of Zircon was studied with magnesium oxide and with magnesiumhydroxidecarbonate. The basic idea was to bind the water insoluble silica with magnesium to a form which is acid insoluble, so as to decrease the silica values in nitrate solutions.

2.2.1 Caustic Fritting with magnesiumhydroxide-carbnonate.

The fritting was done at different temperatures with magnesiumhydrodixecarbonate to sodium hydroxide to zircon ratio of 4.5:11:10. The preheating time of

o 120 minutes was selected as in many previous works . The results are given in Table III. The frit was amorphous in nature and flowed rather easily from the crucible. Due to amorphous nature of the products different components of the frit could not be identified by XRD technique. The frit had to be dried before dissolving it in nitric acid. Incomplete drying hampers the recovery of soluble zirconium and the filteration of nitrate is also extremly slow. At higher fritting temperature silica removal was more effective but the % of soluble zirconium lowered.

2.2.2 Caustic Fritting with Magnesium Oxide

The fritting was done at different temperatures with a fixed ratio of magnesium oxide to caustic to zircon as 3:11:10. Data are given in Table IV. The frit was amorphous in nature and flowed from the crucible very easily. The products could not be identified by XRD, because of noncrystalline nature. The frit was dried prior to dissolving it in nitric acid. Incomplete drying results in slow filteration due to the formation

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of a collodial state. Silica removal is also found to be less sufficient. The temperatures required for fritting with magnesium oxide are much less than that with magnesiumhydroxidecarbonate. Silica removal is also better. To optimize the amount of magnasium oxide, a series of experiments was done with varying amounts of magnesium oxide. Data are given in Table V. Magnesium oxide to zircon ratio of 2.2:10 appears to be the optimum with zircon to caustic ratio of 1:1.1.

3. DISCUSSION

The drying of washed frit prior to nitric acid dissolution appears to be very important in subsequent steps. The incomplete drying hampers the dissolution efficiency, silica removal and rate of filteration. The drying of washed frit appears to dehydrate the silica and convert it, to a form that is less soluble in nitric acid.

The temperatures required for magnesiumhydroxidecarbonate fritting were high, resulting in high corrosion rate of crucible. The silica removal was also insufficient.

Experiments show that stirring is necessary at the stage of boiling of melt. The reason for this was thought to be poor mixing in this small dia crucible where natural agitation is sufficiently inhibited. It is hoped that in a large dia crucible where eddi currents are formed, the problem of stirring will not arise.

The data shows that friti.imi wtfh icjijm'u mm oxide? has removed much of silica to bring it to a level of less than 1000 ppm. However the magnesium effects to reduce the amount of silica in the nitrate solutions is neither studied in these investigations nor completely understood. It is postulated that following reactions play a major contrilbution role.

6 NaOH + Zr SiO^—^Na 2 Zr03 + Na.SiC^ + 3H.0

4 NaOH -f Zr Si0 4—^Na 2 ZrO-j + Na 2Si0 3 • 2H20

However as some of the reacted silica in the normal caustic fritting is water insoluble but acid soluble which is believed to be product of some minor reactions taking place simultaneously.

12 D'Ans has suggested that formation of sodium zirconium silicate as one of the possibilities, which may be expressed by the

following equations:

Na_ Zr03 + Na2Si_05—^Na_Zr Si05 • Na 2Si0 3

Na. 6x0. * Zr SiO.—^-Na-Zr Si05 • Na 2Si0 3 It appears that presence of magnesium oxide supreses the reactions that lead to the formation of water insoluble but acid soluble silicates. This may be due to the reaction of magnesium oxide/ magnesium hydroxide carbonate with sodium zirconium silicate forming ortho and/or meta silicates of magnesium, which are insoluble in water as well as in nitric acid.

Na-Zr SiO, + MgO—^Mg Si0 3 • Na_Zr03

Na2Zr Si05 + MgO—>Mg 2Si0 4 + Na 2Zr0 3

The reduction in % recovery of soluble zirconium at higher temperatures can be attributed to the formation of complex silicates like magnesium zirconyl silicates (Mg ZrSiO,-) and magnesioz:rcono/

13-14 silicate (4 MgO. Zr0 2. Si02) . The present investigations have helped to develop a new method of fritting which will eliminate sulphuric acid dissolution and subsequent precipitation with ammonia and washing of sulphate ion from gelitinous hydroxide cake. It is hoped that savings in terms of sulphuric acid and ammonia will be much more than cost of magnesium oxide used in fritting.

4- SUMMARY AND CONCLUSIONS

1. The magnesium oxide/magnesiumhydroxidecarbonate participate in the fritting reactions at elevated temperatures.

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The magnesium probably binds silica by forming nitric acid insoluble meta and/or ortho silicates.

As regards silica removal, magnesium oxide is more effective than magnesium hydroxide carbonate, and the temperature required for fritting with magnesium oxide is lower.

The frit formed is amorphous and flows easily from the crucible.

Due to amorphous nature, reaction products could not be identified by XRO.

The complete drying of frit is crucial in filteration and removal of silica.

2.2:10 magnesium oxide to zircon ratio seems to be the best in terms of silica removal and soluble zirconium recovery.

At higher temperatures (750°C) % recovery of soluble zirconium is lowered both in the case of magnesium oxide and magnesium hydroxide carbonate. 725°C seems to be the optimum temperature for fritting with magnesium oxide.

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ACKNOWLEDGEMENT

The authors are grateful to Director PINSTECH and Head NMD for provision of facilities and encouragement. They are also thankful to Mrs. Farhat Waqar for help in silica analysis and to H/s M. Istiaq and H. Javed Cill for their assistance.

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REFERENCES

1. ASTM, B 349-67. 1973 Part-7. 2. R-Lustman, B and F. Kerze, Jr : The Metallurgy of Zirconium,

Mc Graw - Hill, 1955 PP. 115 - 129. 3. B Na'di et al. Solvent Extraction and Ion Exchange (R)

1(1), 141 - 202 (1983). 4. Sajin, N.P. and D.A. Papelyaeva: PICG (1) 8 : 559 (1956). 5. Mechlain, JH and S.M. Shelton "Zirconium Hafnium

Seperation". In the Reactor Hand book, 2nd ed. Vol-1 Materials, C.R. Tipon (ed), interscience, New York, 1960.

6. COX, R.P. et al : Ind. Eng. Chem. 50. 141 (1958). 7. Spink, D.R. and Willhelm, H.A.U.S.

Atomic Energy Commission, Report I.S.C. 217, March, (1952).

8. CHOI, H.S. Transactions, Volume LXVIII, 1965, (PP 65 -70).

9. Aniel K. Mukherji. "Analytical Chemistry of Zirconium and Hafnium", p.184 Ed. 1970.

10. Korbi, J. and pribil, R., Chemist Analyst, 45>, 102 (1956). 11. ASTM, E-146 - 68. 1973 Part 32. 12. D'Ans, J., and Loffer, J., Z. Znorg.

U. Allgem. Chem., 191, 1930, 22. 13. A.L. Roussin and J.H. Chesters, Trans,

Ceram. Soc (England), _3£> 217 - 24 (1931). 14. W.J. Rees and J.H. Chesters. Trans.

Ceram. Soc (England), 2±, 309 - 16 (1930).

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Table 1: Requirements of Zirconium Sponge Reactor Grade R-1

Permissible Impurities Element max; ppm.

Aluminium 75 Boron 0.5 Cadmium 0.5 Carbon 250 Chlorine 1300 Chromium 200 Cobalt 20 Copper 30 Hafnium 150 Iron 1500 Manganese 50 Nickel 70 Nitrogen 50 Oxygen 1400 Silicon 120 Titanium 50 Tungsten 50 Uranium (Total) 3.0

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Table 2: The Chemical Composition of Pakistani Zircon.

Zr02 + Hf0 2 Silica Ferric Oxide Titanium Oxide Aluminium Oxide

66.3% 32.891% 0.069% 0.42% 0.22%

Table 3: Effect of Temperature on Si0 2 Contents in Final Zr (N0 3). Soln. (Mg C0 3 Fritting)

S/ No.

Fritting Temp.

Soluble Zirconium Recovered (%)

SiO -2 x 100 Zr

1.

2.

775

825

83.9

74.56

1.39

0.364

875 85.3 0.258

REACTION PARAMETERS

Heating Time: 120 Minutes. MgC03 : NaOH: Zircon 4.5:11.:10,

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Table 4: Effect of Temperature on SiO- Contents in Final Zr(NO~). Soln. (MgO fritting). ^ J *

s/ Fritting Temp.

Soluble Zirconium Recovery (%)

SiO- x 100 Dryinc Washed

j Temp, of NO.

Fritting Temp.

Soluble Zirconium Recovery (%) Zr

Dryinc Washed I Frit (C°;

1. 725 80.4 0.219 95 2. 725 81.6 0.274 100 3. 725 84.4 0.114 150 4. 775 77.1 0.101 100 5. 775 79.3 0.298 100 6. 825 69.8 0.298 100 7. 875 65.1 0.248 100

REAC^ PI ON PARAM1 STERS

Heating Time: 120 Minutes MgO : NaOH : Zircon = 3:11:10.

Table 5: Effet of Varying MgO Ratios on Soluble SiO- in Final 2r (N0 3) 4 Soln. z

s/ No.

MgO : NaOH : Zircon Soluble Zirconium Recovered (%)

SiO- Xj00 Zr

1. 3 : 11 : 10 77.5 0.202 2. 2.5 : 11 : 10 65.9 0.059 3. 2.2 : 11 10 25.0 0.017 4. 1.65: 11 : 10 63.57 0.298 5. 1.1 : 11 • 10 78.807 0.191 6. 0.55: 11 • 10 78.55 0.45

REACTION PARAMET1 3RS

Fritting Temp. * 725°C Heating Time * 120 Minutes.